Speaking with Stuart Phillips at McMaster University in Ontario, he explains why the squat is the one weight training exercise everyone should do:

I do think, from top to bottom, that the squat is an exercise that engages every single major muscle group... It's about as functional as you're going to get in terms of training you for activities like picking things up and standing up.

The key word here is "functional". While most weight training is about bulking up for activities more extreme than you probably encounter on a daily basis, squats train you for what really matters: your day-to-day. Thankfully, you don't even need a set of weights for squats and you can do them pretty much anywhere.

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Also, If you want to burn calories, the bigger the muscle , the more calories burnt. The muscles in your leg that work hard to do the squat are some of the biggest strongest muscles in your body = lots of calories burnt. Also also, they can push heaps of weight = even more calories burnt.

Downside - after a good set of sqauts, you can't sit down without using your arms to lower yourself for at least a week.